Choptank River

In this file photo, sea gulls sit on the frozen shore of the Choptank River in Cambridge, Wednesday, Jan. 14, 2015. Federal data released Friday, Jan. 23, shows that the amount of pollution flowing from nine major rivers into the Chesapeake Bay in 2013 was below a 25-year average.

EASTON — Federal data released Friday, Jan. 23, shows that the amount of pollution flowing from nine major rivers into the Chesapeake Bay in 2013 was below a 25-year average.

But most of the Bay’s tidal water remains unhealthy, according to the Chesapeake Bay Program.

“We have made significant progress in implementing our nutrient and sediment pollution control practices since the pollution diet, or TMDL, went into effect in 2010,” said Bay Program Director Nick DiPasquale.

The Chesapeake Bay Program measures achievement of water quality standards to see changes in Bay health over time, and also tracks pollution loads into rivers and streams to ensure its partners are on course to meet water quality goals of the Chesapeake Bay Watershed Agreement. By reporting on both environmental indicators together, the Bay program said it gains a full picture of the watershed’s health.

DiPasquale said the water quality results released last week, which are based on actual water quality monitoring data, “make it clear there is more work to do.”

Between 2011 and 2013, 29 percent of the water quality standards necessary to support underwater plants and animals were achieved, according to U.S. Geological Survey data. That’s a 2 percent drop from before, and is likely a result of weather-related challenges underwater grasses have faced over the last five years, according to the Bay program.

However, scientists expect the lower pollution levels to have a positive impact on the long-term health of the Bay. The drop in nitrogen, phosphorus and sediment pollution loads in 2013 is credited to below-average river flow and pollution-reduction practices implemented on land.

According to the Bay program, continuing to lower the amount of nutrient and sediment pollution moving into the water is critical to restoring the Bay, which is why the program tracks pollution loads and trends in rivers and water quality progress in the Bay.

“We are witnessing an ecosystem in recovery. We must accelerate our effort to make certain we succeed in achieving our water quality objectives,” DiPasquale said.

Nitrogen, phosphorus and sediment are among the leading causes of poor Bay health.

Nitrogen and phosphorus can fuel algae blooms, which leads to low-oxygen “dead zones” in deep water and short-duration “mortality moments” in shallow water, according to the Bay program. Sediment can block sunlight from reaching underwater grasses and suffocate shellfish.

According to the program, intensified urban, suburban and agriculture development can push pollution into rivers, streams and the Bay. Pollution-reducing practices — updating wastewater treatment plants, lowering vehicle and power plant emissions, reducing fertilizer use on lawns and installing conservation practices on farmland — can lower environmental impacts.

According to the USGS, nutrient and sediment pollution loads measured in water between October 2012 and September 2013 were below long-term averages from 1985 to 2013.

However, though concentration trends in nitrogen, phosphorus and sediment pollution have improved in some of the rivers tested by USGS, the last decade has seen no improvements in phosphorus or sediment concentration, with several rivers showing degrading conditions, according to the Bay program.

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